This invention is in the field of burners; more particularly, the invention relates to high temperature burners suited for use in soaking pits for heating partially chilled or cold steel ingots to a uniform temperature for rolling.
In the operation of soaking pits, a number of ingots are ordinarily charged into the heating chamber and removed after they have attained proper temperature. The heating chamber is heated by hot combustion gases from a burner which is generally located in the wall of the upper portion of the chamber. The combustion gases travel across the top of the chamber and then back through the lower portion and out through exhaust ports located in the wall of the lower portion of the chamber.
It is desirable to utilize the heat in the flue gases to preheat combustion air for the soaking pit combustion system. This is accomplished by having suitable recuperators through which the flue products pass in a heat transfer relationship with incoming combustion air.
The flue gas leaving the combustion and heating chamber of the soaking pit furnace is on the order of magnitude of 2400.degree. F. Recuperator systems are known in the art which can withstand the deleterious affects of the high temperature flue gas and capture the heat carried within the flue gas for use in the soaking pit furnace. A discussion of the prior art concerning soaking pit and recuperator systems will be helpful in understanding the present invention.
U.S. Pat. No. 3,107,904 by W. H. Dailey, Jr. and U.S. Pat. No. 3,199,852 by F. J. Koinis et al describe soaking pit systems of interest. The soaking pit systems described in these patents contain certain basic features including: a heating chamber, a recuperator through which the flue gas passes heating secondary combustion air; a stack through which the flue gas leaves the system; a source of primary air; a means to heat the primary air; a burner; a jet pump through which the primary air blows to the burner, drawing secondary air through the recuperator; and a means to control the ratio of primary to secondary air. The soaking pits described in these two patents use a tile recuperator which is common to a plurality of heating and combustion chambers. However, in both of these patents the jet pump or venturi tube is separate from the burner.
U.S. Pat. No. 2,849,221 by C. Cone describes a heat treating furnace in which there is a means to recirculate heating chamber gases when the heating burner is off. A second burner is fired into an auxiliary chamber and gases at high velocity from the chamber pass through an opening duct and then into tubes which lead into the heating chamber of the furnace. These high velocity gases draw some of the exhaust gases from the heating chamber through the opening duct by an inspiration effect and recirculate them through the tubes back into the furnace chamber. The remaining exhaust gases from the furnace chamber pass through a tile and a metal recuperator and then out of stack. The recirculated furnace gases in this patent are not brought into contact with the second burner. The primary air passing through a jet pump draws secondary air through a tile recuperator, entrains it and carries it to the heating burner. The secondary air is drawn by the jet pump, which is discrete from the heating burner, and is not drawn directly into the heating burner or second burner.
U.S. Pat. No. 2,991,832 by W. H. Dailey, Jr. describes a heat treating furnace in which exhaust gases from the soaking pit of the furnace pass through a tile recuperator and then a metal recuperator and finally exit an exhaust stack. Primary combustion air is delivered, under pressure, to the intake side of the metal recuperator by a motor driven blower. The primary air passes from the metal recuperator to a jet pump which is separate from the burner. The jet pump draws secondary air through the tile recuperator and entrains it with the higher pressure primary air. The combustion air passes to a burner located on the side of the furnace. The heating flame enters the furnace chamber from a firing port which preferably has a venturi construction. There is a second jet pump open to the passage connecting the exhaust flue with the firing port. The inlet of the firing port is coincident with the outlet of the burner which has an independent supply of combustion air and fuel. A portion of the primary combustion air is delivered to the firing port through the second jet pump which is in communication with the exhaust flue. The primary air is introduced into the second jet pump through a nozzle and draws part of the products of combustion through the exhaust jet pump to the firing port by the inspirating effect of the high velocity of air discharged through the nozzle. Thereby the furnace atmosphere is recycled as the burner is turned down. The burner is not designed or used as a jet pump to draw secondary air through the recuperator and to the burner, itself or for mixing the primary air with the secondary air.
U.S. Pat. No. 2,689,722 by P. L. Knight also shows a heating apparatus for soaking pits. A plurality of soaking pit heating and combustion chambers are shown each having a separate recuperator. U.S. Pat. No. 2,171,353 by H. A. Brassert is a method for utilization of waste heat using two recuperators in series, a tile recuperator and a metal recuperator. This patent notes that the waste gases from soaking pits are at much higher temperatures than the available metal tube recuperators can withstand. Gaseous material to be heated to higher temperatures pass through the tile recuperator cooling the flue gases passing through the tile recuperator to the metal recuperator. Gases to be heated to lower temperatures pass through the metal recuperator to capture heat from the cooler flue gases passing through the metal recuperator to an exhaust means.
Soaking pit furnace systems are known which use both tile and metal recuperators disposed within an exhaust passage from the heating chamber which leads to an exhaust means. Such furnaces have burners which receive combustion air from a jet pump means. Lower temperature, higher pressure, primary combustion air is heated in the metal recuperator and fed through the jet pump to the burner at a relatively high velocity. This primary air draws lower pressure secondary air through the higher temperature tile recuperator and entrains it within the combustion air stream which is fed to the burner. The jet pump is made of metal and like the metal recuperator and cannot stand the high temperatures of the waste gases coming out of the soaking pit. Therefore, there is a limit to the temperature of the high temperature secondary air and the ratio of the primary and secondary air which can go into the metallic jet pump. As a result, the temperature of combustion air going to the burner is limited and, therefore, the temperature of the flame is limited.
The separate jet pump has been used in order to take advantage of the heat in the secondary air which passed through the tile recuperator. The pressure of the secondary air is limited by the tile recuperator. The secondary air pressure cannot vary greatly from the flue gas pressure in the tile recuperator or the pressure difference across the walls separating the two gases, secondary air and flue gases, will be more than the structure of the tile recuperator can withstand. The pressure necessary for a practical flame in the soaking pit is supplied by lower temperature primary air which passes through a metal recuperator. The metal recuperator can structurally withstand greater pressures but not greater temperatures than a tile recuperator. As indicated, a jet pump has been used in higher pressure, lower temperature primary air lines to draw the lower pressure secondary air through the tile recuperator for mixture with the primary air. Thereby resulting in higher temperature combustion air for a hotter flame. The jet pumps are made of metal and the amount of secondary air which they can draw is, therefore, limited by the same temperature constraints as the metal recuperators.
Thus, there is a need to more efficiently use the heat of the flue gases coming from the heating and combustion chamber of a soaking pit to heat combustion air and thereby result in a higher temperature flame without the use of additional fuel. It is desirable to overcome the limits of combustion air temperature imposed particularly by the metallic jet pump.